Relaying is considered for LTE-Advanced as a tool to improve, for example, the coverage of high data rates for User Equipment (UE), group mobility, temporary network deployment, the cell edge throughput and/or to provide coverage in new cell areas. E-UTRAN supports relaying by having a Relay Node (RN) wirelessly connected to a base station (eNB) (referred to as a Donor eNB (DeNB)). In addition to serving its own ‘donor’ cell, the DeNB serves the RN, via a modified version of the E-UTRA radio interface. The modified interface is referred to as the ‘Un’ interface or the ‘RN-Un’ interface.
Each RN is provided with many aspects of a base station's functionality and is therefore capable of acting as a base station serving user equipment in its own ‘relay’ cell via a wireless interface referred to as the ‘RN-Uu’ interface. From the perspective of the user equipment in the relay cell, therefore, the RN essentially appears to be a conventional LTE base station. In addition to the base station functionality, however, the RN also supports a subset of the UE functionality including, for example, many aspects of the physical layer, layer-2, radio resource control (RRC), and non access stratum (NAS) functionality, to allow it to connect wirelessly to a Donor eNB (DeNB).
The DeNB is capable of handling communication ‘directly’ to and from user equipment camped in its own cell via a conventional ‘Uu’ interface between the DeNB and the user equipment. The DeNB is also capable of handling communication ‘indirectly’ with user equipment camped in the relay cell, via the RN-Un interface, the RN, and the RN-Uu interface.
As those skilled in the art will understand, conventional eNBs have the capability of interconnecting with one another via an interface referred to as the ‘X2’ interface. The eNBs also connect to a core network comprising an Evolved Packet Core (EPC) by means of an interface referred to as the ‘S1’ interface and, more specifically, to a (MME) Mobility Management Entity (MME) of the EPC via an ‘S1-MME’ interface and to a Serving Gateway (S-GW) by means of an ‘S1-U’ interface.
The DeNB is therefore required to provide S1 and X2 proxy functionality between the RN and other network nodes (other eNBs, MMEs and S-GWs) meaning that, depending on the context, the DeNB appears as an MME (for S1), an eNB (for X2) and an S-GW, to the RN. Thus, in addition to terminating the conventional radio protocols of the modified E-UTRA radio interface (RN-Un), the RN is also capable of terminating the protocols of the S1 and X2 interfaces.
When the RN is not serving any UEs (for example at night when fewer UEs are active), however, the RN keeps itself active over both the RN-Un and the RN-Uu interface. This is undesirable because the maintenance of the interfaces expends energy unnecessarily.
The present invention aims to provide an improved communication system and improved components of the communication system, which overcome or at least alleviate the above issues.
The inventors have considered a number of possible ways to achieve this objective but have found that whilst there are existing proposals for Energy Saving Modes (ESMs) to reduce the energy consumption of eNBs and UEs, the implementation of ESMs in the case of RNs is not straightforward.
According to one potential solution conceived by the inventors, an RN reduces energy consumed over the RN-Un interface by moving into a low energy consumption idle mode when it determines that the UE's in its relay cell are inactive (or have remained so for a predetermined period of time). However, whilst this does indeed reduce energy consumption, the transition into the idle mode results in the loss of the S1/X2 context. This is not ideal because it is preferable for the RN to maintain the S1/X2 context in order to continue to receive signals from the DeNB (e.g. acting as an MME (for S1) or an eNB (for X2)) correctly.
According to another potential solution conceived by the inventors, an RN uses a discontinuous reception (DRX) mechanism, similar to that agreed at the 3GPP for implementation in UEs, for reducing the energy consumed over the RN-Un interface. However, whilst this solution also provides significant benefits in terms of reduced energy consumption, it also is not without its issues. These issues arise, in part, because of the need for the RN to support paging from the DeNB to idle mode UEs served by the RN and, in part, because of the need to maintain other radio bearers established for Operations, Administration, and Maintenance (OAM) purposes, which bearers remain present even when there are no active UEs in the relay cell.
More specifically, even where all UEs in the relay cell are in idle mode, a paging message (e.g. a S1-AP: Paging message) can still be received at any time over the Un interface, with one message being received for each paged UE. To receive paging messages, a UE in idle mode monitors the Physical Downlink Control Channel (PDCCH) for a Paging Radio Network Temporary Identifier (P-RNTI) used to indicate paging.
A P-RNTI indicating a paging message may be transmitted at predefined Paging Occasions (POs) within a Paging Frame (PF) (a single radio frame) that may contain one or more Paging Occasion(s). Currently, for example, up to four POs are allowed in each paging frame (PF) for an eNB. When DRX is used, the UE need only monitor once per DRX cycle. If the terminal detects a P-RNTI when it wakes up from its DRX cycle, it will process the corresponding paging message.
The UE derives the PF to monitor using the following formula:SFN mod T=(T div N)*(UE_ID mod N)where:                N=min[T, nB] (i.e. if nB<T then N=nB otherwise N=T);        UE_ID=IMSI mod 1024;        SFN is the Cell System Frame Number;        T is the DRX cycle for the UE;        nB is a parameter broadcast in a system information block (SIB2) which may be equal to a multiple or fraction of T (e.g. nB=4T, 2T, T, T/2, T/4, T/8, T/16, or T/32);        IMSI is the International Mobile Subscriber Identity for the UE; and        UE_ID is a UE identifier based on the IMSI.        
Accordingly, the eNB must be able to send a paging message in every radio frame, and hence the same requirement applies to the RN, over the RN-Uu interface. However, paging delays can occur, if the RN operates in a DRX mode over the RN-Un interface during a period that a paging message for a destination UE in the relay cell is sent by MME to the DeNB. On receipt of such a message, the DeNB must wait until the RN begins to monitor the PDCCH and comes out of DRX, before the paging message can be transmitted to the RN. Then, when the RN has received the paging message, it also has to wait until the destination UE monitor the PDDCH and wakes up. If the UE has missed an opportunity to receive the paging message by a narrow margin, the resulting delay can be particularly significant. Such a delay is undesirable as it can result in a prolonged call set up time (calling party delay). Effectively, allowing the RN to use DRX over the RN-Un interface can result in a doubling, or near doubling, of the paging delay for a UE relative to the delay experienced when the UE is paged directly from a conventional eNB, or from an RN that does not employ DRX.